N-Heterocyclic Carbene Catalysis via the α,β-Unsaturated Acyl Azolium

Abstract

First reported less than a decade ago, the α,βunsaturated acyl azolium has emerged as a central reactive intermediate for reaction discovery using N-heterocyclic carbene catalysis. In this Perspective, an introduction to the four main reactivity patterns accessible from this intermediate is provided. The Perspective is handled in a largely chronological fashion, with an emphasis on alternate approaches to the key intermediate and first-in-class reaction cascades. Finally, a brief discussion of emerging trends in this field of catalysis is presented. Although not exhaustive, the Perspective provides an overview of this active area of research and serves as a guide for future investigations.

Optical and XPS evidence for the electrochemical generation of an N-heterocyclic carbene and its CS2adduct from the ionic liquid [bmim][PF6]

“…[35][36][37][38] Finally, NHCs proved to be highly effective organocatalysts and have been utilized in a wide range of organic reactions. [39][40][41] Traditionally, deprotonation of imidazolium salts using a strong base such as NaH, KOtBu, etc. has been the preferred method for the preparation of the corresponding NHCs (Scheme 1a).…”

N‐Heterocyclic Carbene Catalyzed Concerted Nucleophilic Aromatic Substitution of Aryl Fluorides Bearing α,β‐Unsaturated Amides

“…[21] To realize catalytic CÀCb ond-forming CS N Ar reactions,w e envisioned the use of N-heterocyclic carbene (NHC) catalysts,w hich can be used to generate an ucleophile from ac arbonyl compound or an alkyl halide in ac atalytic manner. [22][23][24] Among the several precursors that can be activated by an NHC catalyst, we opted to use a,b-unsaturated carbonyl compounds,which are known to serve as latent b-carbanions via umpolung.A lthough the generation of this type of nucleophile has been utilized in several NHCcatalyzed reactions [25] since the first report by Fu and coworkers, [26] its use in catalytic aromatic substitution reactions has not been accomplished. [27,28] We used an aryl fluoride bearing an a,b-unsaturated amide,w hich would undergo an aromatic substitution reaction by acatalytically generated bcarbanion (Scheme 1b).…”

N‐Heterocyclic Carbene‐Catalyzed β‐Indolylation of α‐Bromoenals with Indoles

“…To our knowledge, the NHC-bound α,β-unsaturated acyl azoliums have been broadly applied in many organic transformations. [5] These successes may be due to the following factors. First, the readily available starting materials make it feasible to generate α,β-unsaturated acyl azoliums via an in-situ formation.…”